1. Field of the Invention
This invention relates generally to thermal blankets used in a medical setting to deliver a bath of thermally-controlled gaseous medium, such as air, to a patient.
2. Description of the Related Art
Thermal blanket prior art is disclosed in commonly-assigned U.S. Pat. No. 4,572,188 entitled "AIRFLOW COVER FOR CONTROLLING BODY TEMPERATURE" and U.S. Pat. 5,405,371 entitled "THERMAL BLANKET". These two patents describe thermal blankets which include a plurality of communicating inflatable chambers. In these blankets, apertures are formed through blanket base sheets. These apertures open through the base sheets into the chambers. When inflated with warmed air, the pressure of the air in the chambers causes the air flow cover to inflate. The apertures exhaust the warmed air through the base sheets, and the warmed air is contained between the base sheets and the patients. Therefore, these thermal blankets create an ambient environment about the patient, the thermal characteristics of which are determined by the temperature and pressure of the gaseous inflating medium.
Temperature control in humans has important medical consequences. The human body has evolved over several million years to maintain its core temperature within a very narrow range. Thermoregulatory responses such as vasoconstriction, vasodilatation, shivering or sweating occur in response to core body temperature changes as small as +/- 0.1.degree. C. Human cellular functions, biochemical reactions and enzymatic reactions are optimized within this narrow temperature range.
The prior art thermal blankets address the problem of warming a patient in order to treat hypothermia (a core temperature that is less than normal) such as might occur operatively or post-operatively. These thermal blankets have proven themselves to be extremely useful and efficient in the treatment of patients whose core body temperatures might otherwise become undesirably low either during or after a medical procedure, such as surgery.
However, there are circumstances under which a patient should be cooled rather than warmed in order to treat hyperthermia (a core temperature that is greater than normal). Hyperthermia may result from environmental heat stress or from illness. Otherwise normal individuals may suffer hyperthermia when their natural cooling mechanisms, such as sweating, are overwhelmed during heavy physical work in a hot environment. This is usually associated with relatively inadequate fluid consumption that results in inadequate sweating. Heat stress disorders, categorized in ascending order of severity, include: heat cramps, heat syncope, heat exhaustion and heat stroke. Normally, a person will voluntarily stop working well before the onset of heat exhaustion, but competitive athletes or military personnel may push themselves beyond this limit.
Hyperthermia may also be caused by fever associated with illness. Such fever has many causes, including: infection, tumor necrosis, thyroid storm, malignant hyperthermia or brain injury. Brain injuries that cause hyperthermia usually involve the hypothalamus, and may be caused by tumors, stroke, head injury or ischemic brain injury due to cardiac arrest.
The physiologic consequences of hyperthermia span a spectrum of severity with fluid and electrolyte imbalances, increased cellular metabolic rates, and cognitive impairment being at the low end. In the mid-spectrum, motor skill impairment, loss of consciousness and seizures occur. At the high end, the individual suffers irreversible cellular injury, especially of the highly metabolic brain and liver cells, and then finally organ failure and death. Hyperthermia is a thus a condition that, depending on its severity, may require immediate cooling treatment to return the patient's core temperature to normal.
Cooling treatment may also have other important uses. There is a growing body of evidence suggesting that in some situations, mild-to-moderate hypothermia may provide beneficial protection against injury. The protective benefit of hypothermia has been shown when the blood flow to all or part of the brain is interrupted. Brain ischemia due to an interruption of the blood flow may occur during cardiac arrest, surgery on the blood vessels of the brain, stroke, traumatic brain injury or open heart surgery. Cooling the brain before or in some cases after these events occur seems to be protective, and decreases the severity of the ultimate brain damage.
Various apparatus and techniques have been used over the centuries to cool the human body. Cooling technologies can be generally categorized as: conductive, convective, or evaporative. While many technologies have been tried, all are limited in the clinical setting by lack of practicality, difficulty of use, ineffectiveness, and/or excessive power consumption.
Conductive cooling is very effective when accomplished by packing a hyperthermic person in ice, or immersing the person in cool, or even cold, water. While ice is an effective cooling agent, it is painful to the patient, can damage the skin, is frequently not available in large quantities, and is not practical for long term use. Water baths are also effective, but not practical for the comatose or intensive care patient, or for long term use. A less effective, but commonly used, method of conductive cooling involves placing the person on, and/or under, a cold water circulating mattress and/or cover. These devices have chambers with circulating water therein. The water cools the surfaces of the device, which in turn removes heat from the patient wherever the surfaces thermally contact the patient's skin. These devices are generally uncomfortable and heavy, and their thermal contact is frequently inefficient because they are not precisely shaped to the body surface.
Convective cooling consists of blowing room temperature air, or cooled air onto the patient. Convective cooling is the least effective method of cooling, from a thermodynamic point of view. Room temperature air can be blown very inexpensively with a fan. However, its cooling effectiveness is severely limited if the patient is not sweating. Cooled air can be made with a traditional compression or heat-pump air conditioner, or with thermoelectric cooling. Cooled air has also been generated for centuries using the so-called "swamp cooler" principle of vaporizing water into the air stream. The water evaporates into the air, thus cooling the air. The cooled air is then applied to a person.
An example of such a cooler is shown in U.S. Pat. No. 5,497,633 entitled "EVAPORATIVE COOLING UNIT" by Jones et al. Once the air is cooled by any of these technologies, it can be delivered to a person by generally cooling the environment around the person, such as cooling the air in a room. For more efficient convective cooling utilizing less energy, the cooled air can be delivered to a person more effectively by confining the cooling to only the person. This can be accomplished using a convective thermal blanket such as shown in U.S. Pat. Nos. 4,572,188 or 5,405,371, referred to above and incorporated herein by reference. Another convective thermal blanket is shown in U.S. Pat. No. 4,777,802 entitled "BLANKET ASSEMBLY AND SELECTIVELY ADJUSTABLE APPARATUS FOR PROVIDING HEATED OR COOLED AIR THERETO" by Feher. Confined convective cooling has also been shown in the form of a jacket-like device in U.S. Pat. No. 5,062,424 entitled "PORTABLE APPARATUS FOR RAPID REDUCTION OF ELEVATED BODY CORE TEMPERATURE" by Hooker.
Convective cooling removes the stress of environmental heat, but is minimally effective in active cooling. This limited thermodynamic effectiveness is particularly evident when trying to cool patients with fevers. Generally, in order to be cooled by convection, the patients must be anesthetized and paralyzed to prevent heat producing shivering. Further, the thermodynamic inefficiency of convective cooling causes this method of cooling to use considerable electrical power and generate considerable waste heat, both of which can be a problem in the emergency or intensive care situation.
Evaporative cooling is the thermodynamic basis of the highly efficient sweating response. Each gram of water that evaporates extracts 540 calories of heat from the skin of the body being cooled. Because of the very large heat of vaporization of water, large amounts of heat are removed from the body by evaporating relatively small amounts of water. Evaporative cooling has been practiced since the beginning of mankind, simply by wetting the skin or clothing, and letting the wetting agent evaporate. Evaporative cooling is used even today in hospitals, in the form of sponge baths, where the patient is wetted with water, and allowed to dry by evaporation. Sometimes a fan will be blown on the patient to increase the rate of evaporation. While this method of cooling is clearly effective, it is labor intensive, messy, requires the patient to be totally exposed, and is generally not practical for prolonged cooling. Finally, the effectiveness of evaporative cooling is severely limited in high humidity environments.
Therefore, there is a need for a temperature control device, and particularly a thermal blanket, that can accommodate a patient who requires treatment for hyperthermia or requires cooling as an injury prevention mechanism. What is required is an inexpensive covering that cools a patient rapidly and efficiently in a clinical setting, yet which may be easily and conveniently used by medical personnel.